A polymer network is a special type of polymer-polymer composition in which favorable interactions exist between the constituent polymers on a molecular level. Many polymer networks of the prior art utilize covalent bonding between the constituent polymers to establish a permanent network structure. In addition to covalent bonding, interactions which promote the formation of a polymer network include coulombic attraction in the case of polyelectrolyte network complexes, hydrogen bonding in the case of polyether:poly(carboxyvinyl) complexes or van der Waals attractions in case of nonpolar polymers. In addition to these types of interactions, physical interactions, such as entanglement and templating, contribute to the interacting nature of these systems. Because of the nature of these interactions, interpolymer systems may possess unique synergistic properties that none of the constituent polymers alone exhibit.
The capability of one component of a network to influence one or more components of a network during synthesis is known. As an example, a reformed polymer may be used as a template in the polymerization of a second polymer. It has been established that the rate of polymerization and the polymerization molecular weight of poly(acrylic acid) is affected by the template polymer used for template polymerization. Adachi, et al. (Polymer J. 14(12):985-992 (1982)) report that polymerization of acrylic acid in the presence of polyoxyethylene resulted in an interpolymer complex having a ladder-like structure in which each oxyethylene residue forms a hydrogen bond with an acrylic acid residue.
The ability to form polymer:polymer complexes provides a stable composition of two or more polymers, even where the polymers may be otherwise immiscible. Thus, it is desirable to provide polymer network compositions which possess all the properties of constituent polymers, but which have improved stability and compatibility over simple blends of the constituent polymers. It is also desirable to provide polymer network compositions in which a synergistic effect between the constituent polymers impart properties not possessed by the constituent polymers, either alone or in a simple blend.
Tanaka, et al. (U.S. Pat. No. 5,503,893) discloses a polymer network in which the interpolymer attractions are strong enough to permit a three-dimensional polymer network without the use of covalent crosslinking between the constituent polymers. The polymer composition of Tanaka is a gel which exhibits a volume change in response to an external trigger.
Reversible gelling solutions are known. Efforts have been directed to the development of gelatinous drug delivery systems for topical applications and for ophthalmic delivery to the eye. Such reversibly gelling systems are useful wherever it is desirable to handle a material in a fluid state, but performance is preferably in a gelled or more viscous state.
A known material with these properties is a thermal setting gel using block copolymer polyols, available commercially as Pluronic.RTM., which is described in U.S. Pat. No. 4,188,373. Adjusting the concentration of the polymer gives the desired liquid-gel transition. However, concentrations of the polyol polymer of at least 15-20% by weight are needed to produce a composition which exhibits such a transition at commercially or physiologically useful temperatures. Also, solutions containing 15-20% by weight of responsive polymer are typically very viscous even under the lower viscosity state of responsiveness, so that these solutions can not function under conditions where low viscosity, free-flowing is required prior to transition. In addition, these polymer concentrations are so high that the material itself may cause unfavorable interactions during use.
Another known system which is liquid at room temperature, but forms a semi-solid when warmed to about body temperature is formed from tetrafunctional block polymers of polyoxyethylene and polyoxypropylene condensed with ethylenediamine, commercially available as Tetronic.RTM. polyols. These compositions are formed from approximately 10% to 50% by weight of the polyol in an aqueous medium. See, U.S. Pat. No. 5,252,318.
Joshi, et al. in U.S. Pat. No. 5,252,318 reports reversible gelling compositions which are made up of physical blends of a pH-sensitive gelling polymer (such as a cross-linked polyacrylic acid) and a temperature-sensitive gelling polymer (such as methyl cellulose or block copolymers of polyoxyethylene and polyoxypropylene). In compositions including methylcellulose, 5- to 8-fold increases in viscosity are observed upon a simultaneous change in temperature and pH for very low methylcellulose levels (1-4% by weight). See, FIGS. 1 and 2 of Joshi, et al. In compositions including Pluronic.RTM. and Tetronic.RTM. polyols, commercially available forms of polyoxyethylene/polyoxypropylene block copolymer, significant increases in viscosity (5- to 8-fold) upon a simultaneous change in temperature and pH are observed only at much higher polymer levels (&gt;12% by weight). See, FIGS. 3-6 of Joshi et al.
Thus, the known systems which exhibit reversible gelation are limited in that they require large solids content and/or in that the increase in viscosity are less than 10-fold.